In performing digital processing of a grayscale image, a variety of thresholding techniques has been employed in the art. For example, in fixed thresholding, the magnitude of an analog output signal from a pixel sensor is compared against a fixed threshold value. Output signal levels above the fixed threshold level are digitized to one digital value, e.g. a binary one representing black, while levels equal to or below the fixed threshold level are digitized to another digital value, e.g. a binary zero representing white.
Other, more complex, thresholding techniques have also been used in the past. U.S. Pat. No. 4,908,875 (issued to D. Assael et al on Mar. 13, 1990 and hereinafter referred to as the '875 patent) describes an adaptive thresholding technique that is designed to vary the threshold value during image processing in order to compensate for the detrimental effects of a variety of noise sources. An adaptive thresholding technique is used here inasmuch as the magnitude of noise signals usually varies from one sensor to the next and at different locations along a scan line. Specifically, as described therein, an adaptive threshold level is set for each pixel position as a function of a running average of an analog sensor signal to which an offset value is added. The offset value is a function of a global measure of the noise as well as a function of the noise generated by the sensors.
U.S. Pat. No. 4,468,704 (issued to J. Stoffel et al on Aug. 28, 1984 and hereinafter referred to as the '704 patent) describes another adaptive thresholding technique that employs a threshold level which is updated continuously on a pixel-by-pixel basis in accordance with changes in image content. This technique uses a fractional average content algorithm in which a threshold level is set in accordance with an adaptive offset value that is a function of white peak and black valley pixel signals. After each pixel is processed, the white peak pixel signal is updated at each black-to-white transition and in each white region. The black valley pixel signal is updated at each white-to-black transition and in each black region. As such, the offset value and therefore the threshold level are updated in accordance with an algorithm based on the updating of the white peak and black valley pixel signals. The thresholding technique of the '704 patent also reduces the effects of noise.
Although such prior-art thresholding techniques have served their purposes, they have not proved entirely satisfactory under all conditions of service for the reason that considerable difficulty has been experienced in processing those areas of the image that are located adjacent to image edges. Most prior-art techniques require substantial settling times at the start of a data scan before valid data becomes available for adaptive thresholding. In this regard, at the very start of each scan line, adaptive threshold values are usually "seeking" a proper signal level and are therefore not usable. Consequently, those concerned with the development of image processing systems and methods have long recognized the need for providing thresholding techniques that mitigate, inter alia, settling-time problems associated with not having valid data. The present invention fulfills this need.